This invention concerns an adjustable electric thermostat, particularly for home appliances, such as ovens and the like.
For the adjustment of the reference temperature, such thermostats include a shaft having an outside knob, threadedly engaged with respect to the housing of the thermostat, so that the shaft end changes its position as the knob is rotated, thus displacing the home position of a temperature transducer comprising an expansible bellows attached to the shaft end. The bellows is filled with a suitable fluid and is connected through a long capillary tube to a remote bulb acting as a temperature probe. Expansion of the fluid inside the bulb causes the bellows to expand and push down the blade of an electric switch to make or break a contact, thus turning on or off a heating circuit.
The thermostat is calibrated by maintaining its housing at normal temperature, say 20.degree. C., and placing only the bulb in the heated environment. However, when the thermostat is used in an appliance such a cooking oven, the entire enclosure of the oven, in which the thermostat is mounted, becomes heated to a considerable extent. Therefore the entire housing of the thermostat, including the bellows, is affected by the temperature of the oven, and its inherent thermal expansion is added to the expansion of the fluid in the bulb, thus giving rise to an overcorrection. In the displacement-temperature diagram of the thermostat, this amounts to a parallel translation of the diagram. Prior atempts to solve this problem have consisted in incorporating in the thermostat a correcting mechanism which is sensitive to the temperature of the housing and acts in an opposite direction to the bellows. Since the displacement of the actuating mechanism in this kind of thermostat is in the order of 0.01 mm/.degree.C., and the temperature drift can be as high as 30 to 40.degree. C. when the measured temperature is 150.degree. C., it turns out that the correcting mechanism must have an extremely small displacement, such as 0.002 mm/.degree.C. Such a small displacement is difficult to achieve correctly, and temperature-compensated thermostats of the prior art have consequently been complex, bulky, and considerably more expensive than thermostats not provided with temperature compensation.